Influence of internal variability on population exposure to hydroclimatic changes

Area of Science:

• Climate Science
• Hydrology
• Environmental Science

Background:

• Future freshwater availability and human water demand face uncertainties from consumption patterns and internal climate variability.
• Internal climate variability can obscure climate-driven hydroclimatic changes over decadal and continental scales.

Purpose of the Study:

• To quantify the likelihood of global population exposure to increased hydroclimatic deficits by 2060.
• To assess the impact of irreducible uncertainty from internal climate variability on water resource projections.

Main Methods:

• Utilized three population growth projections and a large ensemble Earth system model.
• Assumed stationary per capita water consumption to define hydroclimatic deficits (evapotranspiration exceeding precipitation).
• Quantified probabilities of population exposure to increased hydroclimatic deficits.

Main Results:

• By 2060, 31-35% of the global population may face >50% probability of hydroclimatic deficit increases exceeding storage; up to 9% face >90% probability.
• Internal variability creates substantial uncertainty, with 86-91% of people living in regions where uncertainty spans both deficit increases and decreases.
• Changes in exposure to large deficits can range from -3% to +6% of the global population due solely to internal variability.

Conclusions:

• Irreducible uncertainty from internal climate variability is critical for climate risk management and adaptation.
• Accurate risk assessment requires accounting for the full spectrum of potential hydroclimatic changes, not just forced trends.
• Under-sampling internal variability in water resource projections can lead to significant misestimations of future water stress.